The mitotic checkpoint is specified by an evolutionarily conserved group of six proteins whose functions are to prevent cells with unaligned chromosomes from prematurely exiting mitosis. The mechanisms by which these proteins detect misaligned chromosomes, and then generate and transduce an inhibitory signal throughout the cell to block the Anaphase Promoting Complex/cyclosome (APC/C) from promoting mitotic exit are major questions that remain to be solved. A major focus of this proposal is to examine the biochemical mechanism(s) by which the mitotic checkpoint inhibits the APC/C. To accomplish this, we have developed a somatic cell-free system that allows us to identify and characterize factors responsible for inhibiting the APC/C. Amongst the factors to be studied is the Mitotic Checkpoint Complex (MCC), a complex consisting of checkpoint proteins hBUBR1, hBUB3, Mad2 and Cdc20, that is the most potent inhibitor of the APC/C reported to date. We discovered that formation of the MCC is linked to mitotic entry and exit and is critically dependent on another checkpoint protein, hMps1 kinase. In addition, we identified a new subunit in the MCC. Ro52 is an autoantigen that is present in patients with Sjogren's syndrome. Ro52 is itself an E3 ubiquitin ligase that may regulate the function of the MCC by modifying some of its subunits. We propose to study the regulation of MCC by focusing on hMps1 and Ro52. We will characterize how hMps1 is regulated in mitosis and how hMps1 contributes to MCC assembly in vivo and in vitro. Preliminary data show that MCC purified from mitotic cells exhibits ubiquitin ligase activity and that BubR1 is one of the substrates within the MCC. We therefore plan to test if Ro52 is responsible for this ubiquitin ligase activity in MCC and whether this modification affects MCC composition and thus function in vitro and in vivo. The possibility that the mitotic checkpoint is regulated by a ubiquitin ligase is supported by recent studies that showed the antagonistic actions of ubiquitin ligase and deconjugase regulates the ability of checkpoint proteins to inhibit the APC/C. PUBLIC HEALTH RELEVANCE: The broad objective of our lab is to understand the key mechanical and regulatory events that specify accurate chromosome segregation in human cells. This is directly relevant to human health as defects in this process results in chromosome imbalance that can lead to tumor formation or developmental defects. This topic is also relevant to understanding how cancer cells survive treatment with drugs such as paclitaxel that are commonly used in the clinic.